1. Field of the Invention
The present invention is concerned with a fault-indicating impedance-changing circuit installed into a load, in particular but not exclusively a light-emitting-diode (LED) lamp.
2. Brief Description of the Prior Art
Incandescent lamps are conventionally used in traffic lights. As well known to those of ordinary skill in the art, a traffic light also includes a conflict monitor (a) to detect turning on of the lamp when it should be turned off and (b) to sense the impedance of the filament of the incandescent lamp to detect failure of the lamp. This conflict monitor is usually designed to operate with incandescent lamps having a rated nominal power of the order of 150 watts and an impedance lower than 1500 xcexa9.
When the power switch through which the incandescent lamp is turned on and turned off is open, a small current is supplied to the filament of the incandescent lamp through a shunt impedance element connected in parallel to the power switch. Upon selecting the impedance of the shunt impedance element, the two following factors are taken into consideration:
the impedance of the incandescent lamp is lower than 1500 xcexa9; and
the voltage measured across the incandescent lamp must not exceed a given voltage threshold when the lamp is in good condition, since detection of a voltage amplitude across the lamp higher than this given voltage threshold indicates a failure of the lamp.
A newly developed technology enables production of LED lamps that meet with the traffic signalling standards regarding light intensity. These LED lamps consume an electric power as low as 20 watts.
However, replacement of an incandescent lamp by a LED lamp raises the problem that light emitting diodes must be supplied with direct current and the input impedance of the required ac-to-dc power supply, included in the LED lamp, is high (when the LED lamp is turned off) if compared to the impedance of the filament of an incandescent lamp.
The easiest solution to the above discussed problem is to permanently connect an impedance element lower than 1500 xcexa9 in parallel to the LED lamp. However, this solution is itself the source of the following problems:
a resistive impedance (resistor) will increase the level of electric power consumed by the LED lamp;
a reactive impedance (capacitor and/or inductor) will reduce the power factor of the lamp; and
in case of a failure of the light emitting diodes and/or the ac-to-dc power supply, the impedance of the LED lamp does not change sufficiently to allow the conflict monitor to detect a fault.
Upon detection of a fault, namely turning on of a lamp when it should be turned off or failure of a lamp, the conflict monitor activates a safety system to cause all the red and yellow lamps of the traffic light to flash for thereby warning the automobilists crossing the corresponding junction.
As road safety must not be neglected, LED lamps must be designed to enable the conflict monitor to detect a fault and activate the safety system in view of warning the automobilists.
An object of the present invention is therefore to overcome the above discussed problems by providing a fault-indicating impedance-changing circuit usable in a LED lamp.
More specifically, in accordance with the present invention, in an electric load, there is provided an input impedance-changing circuitry for establishing a low input impedance circuit of the electric load when this electric load is turned off. The input impedance-changing circuitry comprises (a) a shunt circuit section including a low impedance element and a controllable switching device connected in series, and (b) a detector circuit section for detecting turning off of the electric load and for closing, in response to detection of turning off of the electrical load, the controllable switching device. Closure of the switching device establishes a shunt circuit including the low impedance element and thereby establishes the low input impedance circuit of the electric load.
Also according to the present invention, in a light-emitting-diode lamp, there is provided an input impedance-changing circuitry for establishing of low input impedance circuit. the light-emitting-diode lamp when this light-emitting-diode lamp is turned off, comprising a shunt circuit section including a low impedance element and a controllable switching device connected in series, and a detector circuit section for detecting turning off of the light-emitting-diode lamp and for closing, in response to detection of turning off of the light-emitting diode lamp, the controllable switching device. This establishes a shunt circuit including the low impedance circuit element and thereby establishes the low input impedance circuit of the light-emitting-diode lamp.
Further in accordance with the present invention, there is provided a traffic-light lamp system comprising:
a light-emitting-diode lamp comprising an input impedance-changing circuitry for establishing a low input impedance circuit of the light-emitting-diode lamp when that light-emitting-diode lamp is turned off, this input impedance-changing circuitry comprising:
a shunt circuit section including a first low impedance element and a first controllable switching device connected in series; and
a detector circuit section for detecting turning off of the light-emitting-diode lamp and for closing, in response to detection of turning off of the light-emitting-diode lamp, the first controllable switching device in order to establish a shunt circuit including the first low impedance element and thereby establish the low input impedance circuit of the light-emitting-diode lamp; and
a second controllable switching device interposed between a source of electric power and the light-emitting-diode lamp for selectively turning on and turning off the light-emitting-diode lamp.
Establishing a low input impedance circuit in a light-emitting-diode lamp when turned off prevents a conventional conflict monitor of a traffic-light lamp system to detect failure of the light-emitting-diode lamp through detection of a high impedance of that lamp.
According to a first preferred embodiment:
the second controllable switching device comprises a power switch for supplying, when this power switch is closed, the light-emitting-diode lamp with electric power from the above mentioned source and thereby turning on the light-emitting-diode lamp, and a second impedance element connected in parallel with the power switch;
the source of electric power is an ac source, the light-emitting-diode lamp further comprises (a) a set of light emitting diodes, (b) a rectifier circuit section supplied with ac voltage and current from the ac source through the first controllable switching device and having an output for delivering rectified voltage and current, and (c) a power converter supplied with rectified voltage and current from the rectifier circuit section for producing dc voltage and current supplied to the set of light emitting diodes, and the shunt circuit section is connected between the output of the rectifier circuit section and the ground and is therefore supplied with rectified voltage and current from the rectifier circuit section; and
the shunt circuit section comprises a resistor forming the first low impedance element, a capacitor and the first controllable switching device connected in series.
In accordance with a second preferred embodiment of the present invention:
the detector circuit section comprises a comparator having a first input supplied with a predetermined voltage threshold, a second input supplied with a voltage across the capacitor, and an output for delivering a first signal when the amplitude of the voltage across the capacitor is lower than the predetermined voltage threshold, this first signal being indicative of turning off of the light-emitting-diode lamp and being supplied to the first controllable switching device to close that first switching device;
the set of light emitting diodes comprises a plurality of subsets of serially interconnected light emitting diodes, these subsets of serially interconnected light emitting diodes being connected in parallel;
the detector circuit section detects a dc current flowing through each subset of serially interconnected light emitting diodes and produces a second signal when no dc current is flowing through a predetermined number of subsets; and
the detector circuit section detects the amplitude of the dc current supplied to the set of light emitting diodes when the power switch is closed, produces a third signal when the amplitude of the dc current supplied to the set of light emitting diodes, when the power switch is closed, is higher than a predetermined current threshold and, in response to the second and third signals, prevents the first signal to reach the first controllable switching device to close that first switching device.
In accordance with a third preferred embodiment of the subject invention:
the detector circuit section comprises a comparator having a first input supplied with a predetermined voltage threshold, a second input supplied with a voltage across the capacitor, and an output for delivering a first signal when the amplitude of the voltage across the capacitor is lower than the predetermined voltage threshold, this first signal being indicative of turning off of the light-emitting-diode lamp and being supplied to the first controllable switching device to close that first switching device;
the detector circuit section (a) detects the amplitude of the ac voltage supplied to the rectifier circuit section when the power switch is closed, and produces a second signal when the amplitude of the ac voltage supplied to the rectifier circuit, when the power switch is closed, is higher than a predetermined voltage threshold, (b) detects the amplitude of the dc current supplied to the set of light emitting diodes when the power switch is closed, and produces a third signal when the amplitude of the dc current supplied to the set of light emitting diodes, when the power switch is closed, is higher than a predetermined current threshold; and
in response to the second and third signals, prevents the first signal to reach the controllable switching member to close that switching member.